memdev: remove "id" property

The "id" property is unnecessary and can be replaced simply with
object_get_canonical_path_component.  This patch mostly undoes commit
e1ff3c67 ("monitor: fix qmp/hmp query-memdev not reporting IDs of
memory backends", 2017-01-12).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>